In the United States, the Federal Communications Commission (FCC) has established regulatory standards for the broadcast of amplitude-modulation (AM) signals in the AM broadcast band (535-1605 kiloHertz). Included in its regulations is one specifying the bandwidth which an AM signal is permitted to occupy, and setting limits for the permissible amount of radiation which may be emitted at frequencies outside the allocated bandwidth. The purpose of this regulation is to prevent each AM station from interfering with the service areas of stations operating at assigned frequencies above and below that station. Compliance with FCC regulations in this respect has required diligence on the part of AM broadcasters and manufacturers of AM broadcast transmitting equipment.
The problem of undesired out-of-band radiation has been compounded with the advent of AM stereo broadcasting. In March 1982 the FCC adopted its unique Report and Order authorizing the so-called "marketplace approach" to the selection of a standard for the broadcast of stereophonic program material in the AM band. In the various AM stereo systems competing in the marketplace, stereo sum information (L+R) is transmitted as amplitude modulation of the carrier, while stereo difference information (L-R) is transmitted as phase modulation of the carrier. The addition of (L-R) information carried in the phase (or quadrature) modulation, particularly for higher-frequency components of L-R, has resulted in an increase in the amount of out-of-band radiation, as compared to monaural AM broadcasting. Furthermore, some AM stereo systems inherently produce more out-of-band radiation than other such systems in the presence of strongly one-sided stereo modulation (L-only or R-only).
Others have recognized that certain AM stereo systems tend to produce more out-of-band radiation, thereby interfering with AM stations broadcasting on adjacent channels, and have attempted to address the problem by processing the L-R signal prior to transmission based on a prediction of when out-of-band radiation would occur (see U.S. Pat. Nos. 4,338,491 and 4,593,402, for example). However, the prediction methods used are generally unreliable and complex.
It is, therefore, an object of the present invention to provide improved automatic means for controlling the out-of-band radiation of an AM transmitter so as to comply with FCC, or other, prescribed limits.